Depends on what kind of airplane you are talking about. In a jet transport in cruise altitude reducing speed by 25% would get you into the low speed buffet/stall region.
Horizontal gusts don't really produce g forces, the main problem is airspeed excursions up, or down, in extreme cases outside of the flight envelope. That's why turbulence penetration speed is somewhere in the middle of the operating speed range, to keep maximum margin to both the low and high speed limits.
As for the vertical gusts - I don't have any reference, but it is basic aerodynamics and some geometry. G-load in a gust is proportional to the square of the speed and to the change of the AoA.
The change in the AoA in turn is proportional to the magnitude of the gust and inversely proportional to the airspeed.
If you combine the two together, the net effect of airspeed will be linear - i.e. twice the speed, twice the g'load for the same gust.